The invention provides chunk polycrystalline silicon and a process for cleaning polycrystalline silicon chunks.
Polycrystalline silicon serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process).
More particularly, however, polycrystalline silicon is required for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for production of solar cells for photovoltaics.
On the industrial scale, crude silicon is obtained by the reduction of silicon dioxide with carbon in a light arc furnace at temperatures of about 2000° C.
This affords “metallurgical grade” silicon (Simg) having a purity of about 98-99%.
For applications in photovoltaics and in microelectronics, the metallurgical grade silicon has to be purified.
For this purpose, it is reacted, for example, with gaseous hydrogen chloride at 300-350° C. in a fluidized bed reactor to give a silicon-containing gas, for example trichlorosilane. This is followed by distillation steps in order to purify the silicon-containing gas.
This high-purity silicon-containing gas then serves as a starting material for the production of high-purity polycrystalline silicon.
The polycrystalline silicon, often also called polysilicon for short, is typically produced by means of the Siemens process. This involves heating thin filament rods of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”), and introducing a reaction gas comprising a silicon-containing component and hydrogen.
In the Siemens process, the filament rods are typically inserted perpendicularly into electrodes present at the reactor base, through which they are connected to the power supply. Every two filament rods are coupled via a horizontal bridge (likewise composed of silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the carrier bodies, which are also called thin rods.
High-purity polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD/gas phase deposition).
After the deposition has ended, these polysilicon rods are typically processed further by means of mechanical processing to give chunks of different size classes, classified, optionally subjected to a wet-chemical purification and finally packed.
The silicon-containing component of the reaction gas used in the deposition of polycrystalline silicon is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I), or a mixture of halosilanes. The halosilane may be a chlorosilane, for example trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
The carbonaceous impurities present in the silicon-containing component of the reaction gas lead to a slight but critically quality-reducing carbon contamination of the silicon deposited therefrom.
This carbon contamination in the silicon bulk is typically up to 50 ppba.
The operations in mechanical processing, up to the packing of the polysilicon, for the most part proceed with full automation. In the course of this, the surface of the silicon particles is contaminated with various metals from the grinding and crushing tools, but also with organic molecules and organic macromolecules (organics).
One cause of the contamination of the polycrystalline silicon with organics is contact, which is not entirely avoidable, of the silicon with components made from an organic polymer or plastic during the mechanical operations.
This surface contamination with carbon compounds is several times higher than the above-described carbon contamination during the gas phase deposition.
This surface contamination of the polysilicon with organics leads to adverse effects for the customer and in customer operations, for example in the production of polycrystalline solar cells:
In the production of polycrystalline solar cells, for example in the block casting process, during the silicon melting and cooling operations, a portion of the organic carbon is incorporated into the polycrystalline silicon block in the form of silicon carbide precipitates. These inclusions lead to adverse effects in the sawing of the sheets as a result of more frequent wire fracture in the wire saw, but also to adverse electrical properties of the material, for example “shunts”, which are ultimately manifested in a poorer efficiency and performance of a solar cell.
For minimization of carbon contamination on silicon surfaces, various methods are known in the prior art.
U.S. Pat. No. 5,445,679A describes a process for cleaning the surface of polycrystalline silicon. In this process, the intention is to transfer organic impurities (unhalogenated and halogenated hydrocarbons) to volatile or gaseous carbon species by contacting with an oxidative atmosphere, and to remove them from the surface. The oxidative atmosphere used is an oxidative plasma which is generated by a conventional high-frequency generator in the plasma gas. The plasma gas used is, according to the oxidation potential required, an inert gas (for example He, Ne, Ar, Kr, Rn or Xe) having an oxygen content of <20%, pure oxygen, pure inert gas, oxygen and halogen, inert gas and halogen, or pure halogen.
However, the apparatus complexity of this process is high since a vacuum tight oxidation reactor with generation of vacuum and plasma is required. Moreover, it is very energy-intensive with comparatively long processing times. In addition, the polysilicon purified by means of oxidative plasma exhibits higher reactivity and hence additional adsorption, such that the plasma-cleaned material in the course of further processing (single crystal pulling) in some cases exhibits higher contamination than non-plasma-cleaned material.
U.S. Pat. No. 4,555,303A describes a process for removing carbonaceous material from, for example, silicon surfaces. This involves exposing the material to be cleaned, in a reactor, to a high-pressure oxygen plasma (in the examples, helium-oxygen mixture having an oxygen content of 32% in each case). The reactive and ionic oxygen species generated in the high-pressure high-frequency plasma lead to a reaction of the carbonaceous layer and hence to oxidation of these impurities. The reaction products, carbon dioxide and possibly a nonoxidized residue (ash material), can be removed readily from the silicon surface with an aqueous solution of sodium hydroxide.
However, the apparatus complexity of this process is high, since a vacuum-tight oxidation reactor with generation of plasma is required. Moreover, the process is very energy-intensive with comparatively long processing times. The wet purification at the end of the process leads to an increased level of complexity and hence to high costs resulting from the purchasing, workup and destruction of chemicals. In addition, the polysilicon cleaned by means of oxidative plasma as known from U.S. Pat. No. 5,445,679A—exhibits higher reactivity and hence additional adsorption, such that the plasma—cleaned material in the course of further processing (single crystal pulling) exhibits higher contamination than a non-plasma-cleaned material.
US 2010154357A discloses a process in which the contamination of chunk silicon is minimized during automatic packing into a plastic bag. To reduce the contamination, including the contamination with carbon, the bags are filled using freely suspended and movable energy absorbers. These energy absorbers for reduction of the momentum of the chunk silicon on the plastic bag consist of a low-contamination plastic. A low-contamination material is understood to mean a material which, after contact with the polysilicon, contaminates the surface thereof to the following maximum extent: carbon less than 300 pptw. This was shown by determining the weight of the tubular plastic before and after the filling of the bags. This showed polymer abrasion (=carbon abrasion) below the detection limit of 0.1 mg per 400 kg, and hence below the required 300 ng per kg of Si (=300 pptw). This process relates only to the limitation of contamination in the course of packing.
US 20030159647A1 discloses “flowable chips” of polycrystalline silicon having a maximum concentration of 0.17 ppma (170 ppba) of carbon in the bulk (bulk contamination). US 20030159647A1 does not give any information as to the concentration of carbon at the surface.
DE 4137521A1 describes a process for analyzing traces of impurities on particulate silicon. This involves introducing a particular mass of the particulate silicon into a silicon vessel, likewise of known mass and quality. This filled silicon vessel is subsequently subjected to a zone pulling process (float zone), which leads to a monolithic unit of monocrystalline silicon between vessel and particulate silicon. The surface impurities of the particulate silicon are thus distributed into the bulk of single crystal formed from the process described. A wafer is subsequently sawn out of the single crystal, chemically cleaned and analyzed. The quantitative analysis of the impurities present in the silicon wafer is conducted by means of standard methods, such as photoluminescence analysis, FTIR spectroscopy and atomic absorption spectroscopy. The concentrations of the impurities of the particulate silicon can subsequently be determined by simple alligation. The content of carbon impurities in sieved silicon fragments is reported as <50 ppb.
DE 4330598A1 describes a process for analysis of traces of impurities in irregularly shaped silicon chunks on the surface and in the bulk. This silicon chunk must have a diameter between 4 and 22 mm and a length of 5 to 20 cm. This involves subjecting a zone-meltable silicon chunk to a crucible-free zone melting process. The surface impurities of the irregular body are thus distributed into the bulk of the single crystal formed from the process described. Subsequently, a wafer is sawn out of the single crystal, chemically cleaned and analyzed. The quantitative analysis of the impurities present in the silicon wafer is conducted by means of standard methods, such as photoluminescence analysis, FTIR spectroscopy and atomic absorption spectroscopy.
The problems described gave rise to the objective of the invention. This was to provide an improved process for removing contamination with carbon at the surface of polycrystalline silicon, and to make available a polycrystalline silicon having less carbon at the surface than in the prior art.